The present invention relates in general to semiconductor power field effect transistors (FETs) and in particular to shielded gate trench FETs with their shield and gate electrodes connected together.
Shielded gate trench FETs are advantageous over conventional FETs in that the shield electrode reduces the gate-drain capacitance (Cgd) and improves the breakdown voltage of the transistor. FIG. 1 is a simplified cross sectional view of a conventional shielded gate trench MOSFET. An n-type epitaxial layer 102 extends over n+ substrate 100. N+ source regions 108 and p+ heavy body regions 106 are formed in a p-type body region 104 which is in turn formed in epitaxial layer 102. Trench 110 extends through body region 104 and terminates in the drift region. Trench 110 includes a shield electrode 114 below a gate electrode 122. Gate electrode 122 is insulated from its adjacent silicon regions by gate dielectric 120, and Shield electrode 114 is insulated from its adjacent silicon regions by a shield dielectric 112 which is thicker than gate dielectric 120.
The gate and shield electrodes are insulated from one another by a dielectric layer 116 also referred to as inter-electrode dielectric or IED. IED layer 116 must be of sufficient quality and thickness to support the potential difference that may exist between shield electrode 114 and gate electrode 122. In addition, interface trap charges and dielectric trap charges in IED layer 116 or at the interface between the shield electrode 114 and IED layer 116 are associated primarily with the methods for forming the IED layer.
The IED is typically formed by various processing methods. However, insuring a high-quality IED that is sufficiently robust and reliable enough to provide the required electrical characteristics results in complicated processes for forming the shielded gate trench FET. Accordingly, there is a need for structure and method of forming shielded gate trench FET that eliminate the need for a high-quality IED while maintaining or improving such electrical characteristics as on-resistance.